In recent years, with emergence of a mobile Internet, a growing quantity of vehicles are equipped with a third generation (3G) or fourth generation (4G) communication function. Compared with a common consumer product, an in-vehicle product has a worse use environment, imposes a higher requirement on reliability, and requires each electronic component in a vehicle to have a fault diagnosis function. An in-vehicle communication function is implemented using a circuit shown in FIG. 1. An external antenna is used to receive and send a radio frequency signal, where the external antenna includes a main antenna and a diversity antenna. A radio frequency coaxial cable is used to transmit a radio frequency signal. An in-vehicle communications device on a vehicle side transmits and receives a radio frequency signal using a radio frequency path. If a user cannot perform communication using the in-vehicle communication function, during detection of a reason why communication cannot be performed, fault determining needs to be performed on the vehicle side, the radio frequency coaxial cable, and the external antenna one by one to finally find a fault location. As a result, complexity and a consumption time of fault diagnosis are increased. Therefore, if the external antenna has the fault diagnosis function, when the in-vehicle communication function fails, steps of detecting the radio frequency coaxial cable and the vehicle side can be avoided when it is known that the external antenna is faulty, or fault diagnosis does not need to be performed on the external antenna when it is known that the external antenna is not faulty.
A fault detection method 1 for an in-vehicle external antenna is as follows. As shown in FIG. 2, a direct current voltage (VCC) is input on a vehicle side, the VCC is connected to one end of a resistor R1, the other end of the resistor R1 is connected to one end of an inductor L, and the other end of the inductor L is connected to a radio frequency path. The VCC does not exert impact on a radio frequency signal transmitted using the radio frequency path and a radio frequency coaxial cable. As shown in FIG. 3, the external antenna includes an antenna board and an antenna body. The antenna board includes a ground terminal. Voltage division can be implemented by connecting, in parallel, a resistor R2, as shown in FIGS. 2 and 3, with a specific resistance value range at an input end of the antenna board. The antenna body is not considered during calculation of a resistance range of the antenna board. As shown in FIG. 2, a voltage detection point is disposed between the resistor R1 and the inductor L on the vehicle side, and a voltage value at the voltage detection point is sampled. If the antenna board operates normally, it indicates that components on the antenna board are all normal. In this case, a resistance value of the antenna board falls within a steady resistance value variation range, and a voltage value detected at the voltage detection point definitely also falls within a steady voltage value variation range. A maximum value and a minimum value of the voltage variation range are set as thresholds. If the antenna board is faulty, it indicates that a component on the antenna board is damaged, and the resistance value of the antenna board is increased or decreased consequently. In this case, the voltage value obtained at the voltage detection point is not between the thresholds such that it can be learned that the external antenna is faulty, and a fault indication is provided.
A fault detection method 2 for an in-vehicle external antenna is as follows. As shown in FIG. 4, the external antenna has a main antenna and a diversity antenna, the external antenna is connected to a detection device, and the detection device includes a detector, a directional coupler, and the like. A communication baseband processor sends an input signal to the main antenna using a transmitter, and the main antenna transmits the input signal. Because an object generally has a feature of reflecting an electromagnetic wave (the input signal is an electromagnetic wave), the main antenna and the diversity antenna may receive a reflected signal obtained by reflecting the input signal by a closest object. The detector of the detection device may obtain the input signal from the main antenna, and may obtain the reflected signal from the main antenna and the diversity antenna. The input signal and the reflected signal are coupled using the directional coupler, and are demodulated and compared to obtain a reflection factor of the external antenna. A smaller reflection factor indicates that more input signals are radiated from the main antenna, and indicates better performance of the external antenna. Because the performance of the external antenna is known, it is set that when the reflection factor is greater than a given threshold, it indicates that the external antenna is faulty, and a fault indication is provided.
However, a potential failure mode exists in the fault detection method 1 for an in-vehicle external antenna, that is, if the antenna body of the external antenna is damaged but the antenna board is normal, overall resistance of the external antenna is not changed, but the radio frequency signal cannot be received or sent, and the voltage value detected at the voltage detection point falls within a normal range. In the potential failure mode, a fault of the external antenna cannot be detected. In the fault detection method 2, because components such as a detector and a directional coupler need to be added, costs are increased. In addition, because the diversity antenna does not have a corresponding transmitter, the diversity antenna does not have an input signal, and can receive only the reflected signal obtained by reflecting, by an object, the input signal transmitted by the main antenna. Consequently, fault detection cannot be performed on the diversity antenna. In this case, when the diversity antenna of the external antenna is faulty but the main antenna is normal, it is detected that the reflection factor of the external antenna is normal, and an obtained result is that the external antenna is normal. However, the in-vehicle communication function fails. Therefore, when the diversity antenna of the external antenna is faulty, no fault indication can be provided.